Resins having improved exchange kinetics

ABSTRACT

Disclosed are novel ion-exchange and chelate-exchange resins having improved exchange kinetics for separating chemical species from liquids. The resins are prepared from copolymer beads consisting of a monovinyl aromatic monomer and a cross-linking monomer. The copolymer beads are functionalized such that weak-base exchange moieties are substituted at haloalkylated sites which are most accessible to diffusion into the beads, while hydrophilic, strong-base exchange moieties are substituted at haloalkylated sites which are least accessible to diffusion. The resins have improved exchange kinetics due to shortened diffusion path lengths for the chemical species being separated and improved diffusion into the resin beads due to the hydrophilic, strong-base exchange moieties.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 07/676,916, filed Mar. 28,1991.

BACKGROUND OF THE INVENTION

This invention concerns ion-exchange and chelate-exchange resins and, inparticular, novel resins functionalized so as to improve exchangekinetics. The invention also concerns a method of preparing the novelresins and a method for their use in separating chemical species fromliquids.

Ion-exchange and chelate-exchange resins are widely employed by industryto separate chemical species from liquids which contain them insolution. These resins are commonly prepared by functionalizing acopolymer bead matrix with functional groups that can associate withchemical species, such as ions or molecules, when the resin is incontact with the liquid. Such resins are generally used in watertreatment and purification, food preparation, pharmaceuticalmanufacturing, chemical processing, metal extraction, and so on, as isgenerally discussed by R. M. Wheaton et al. in, "Ion Exchange", Vol. 11Kirk-Othmer Ency. Chem. Tech. pp. 871-899 (2nd Ed. 1966).

A disadvantage associated with such resins, and widely recognized withinthe art, is slow diffusion into the resin beads for the chemical speciesbeing separated. To attain the maximum operating capacity for the resin,it is necessary to use essentially all available exchange sites withinthe resin bead volume. To do so, substantially all of the availablediffusion path length, i.e., the radius for a fully functionalized resinbead, must participate in exchange with the chemical species. Fullutilization of the diffusion path length in this instance requires arelatively long time to reach exchange equilibrium. In contrast, resinshaving short diffusion path lengths reach exchange equilibrium morerapidly than resins having longer diffusion path lengths. A shorterdiffusion path length therefore allows for more rapid access toavailable exchange sites and a quicker approach to exchange equilibrium.This shortened diffusion path ultimately leads to an ability to processrelatively large amounts of feed streams without unduly sacrificingoperating capacity.

Industry has previously made attempts to shorten the diffusion pathlength by reducing the size of resin beads. However, small beads lead tolarger pressure drops across a resin bed and reduced flow rates for feedstreams being processed. As such, substantially reducing the size of theresin beads is not practical for a commercial process.

Macroporous resins, such as those disclosed by Meitzner et al. in U.S.Pat. No. 4,224,415, were developed to improve kinetics by providing ahighly porous copolymer bead matrix wherein relatively large pore sizesimprove diffusion of chemical species into the interior portions of thebeads. However, these resins also have a considerable amount of exchangesites which are relatively inaccessible to diffusion.

Many mining operations generate aqueous streams containing one or moreheavy metals, like copper or nickel. Industry typically employs twomethods to recover such metals, namely, solvent extraction or the use ofchelate-exchange resins. Traditionally, solvent extraction has been usedto recover such metals, but due to waste disposal considerations thismethod is gradually becoming obsolete. Accordingly, chelate-exchangeresins are becoming important for these applications.

Improved exchange kinetics are particularly desirable forchelate-exchange resins, since diffusion of chemical species is oftenlimiting with respect to the particular chelation reaction involved. Forinstance, U.S. Pat. Nos. 4,031,038 and 4,098,867 disclosechelate-exchange resins derived from aminopyridine compounds, such as2-picolylamines. Although the resins are highly selective for metalslike copper or nickel, they exhibit relatively slow exchange kinetics,i.e., the time required to reach equilibrium capacity for metal loadingis fairly long. As such, a large amount of the resin is needed or only aportion of the available exchange capacity is used, to maintain acommercially reasonable feed rate for the liquid stream being processed.Further, partial use of the exchange capacity is an uneconomical use ofthe resin, since it is relatively expensive to produce.

Accordingly, it is desirable to develop resins which (1) exhibitimproved exchange kinetics without undesirable increases in bed pressuredrop, (2) allow for greater utilization of available exchange capacity,and (3) promote efficient loading and elution of the chemical speciesbeing separated. Such resins would result in a more economical andefficient separation process.

SUMMARY OF THE INVENTION

The above objects and advantages are obtained, in one aspect, by a resinhaving improved exchange kinetics. The resin comprises cross-linkedcopolymer beads having weak-base exchange moieties substituted athaloalkylated sites which are most accessible to diffusion andhydrophilic, strong-base exchange moieties substituted at haloalkylatedsites which are least accessible to diffusion.

Another aspect of the present invention is a process for preparing theresin described in the preceding paragraph. The process comprises thesteps of:

(a) contacting haloalkylated, cross-linked copolymer beads with a firstaminating agent under conditions and for a time sufficient to substituteat least a portion of the haloalkylated sites with the weak-baseexchange moieties, the substitution being conducted at a reaction ratewhich is greater than the rate at which the first aminating agentdiffuses into the copolymer beads: and

(b) contacting the partially functionalized copolymer beads with atertiary amine under conditions and for a time sufficient tofunctionalize at least a portion of remaining haloalkylated sites withthe strong-base exchange moieties.

Another aspect of the invention is a process having improved exchangekinetics for separating chemical species from a liquid which containsthe chemical species in solution. The process comprises:

(a) contacting the liquid with a resin such that the chemical speciesare retained thereby, the resin comprising a plurality of cross-linkedcopolymer beads having weak-base exchange moieties substituted athaloalkylated sites which are most accessible to diffusion andhydrophilic, strong-base exchange moieties substituted at haloalkylatedsites which are least accessible to diffusion: and

(b) eluting the retained chemical species from the resin with aregenerating agent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of conversion versus time for results discussed inconnection with Example 1 and Comparative Example A.

FIG. 2 is a graph of conversion versus time for results discussed inconnection with Example 2 and Comparative Example B.

FIG. 3 is a graph of conversion versus time for results discussed inconnection with Example 3 and Comparative Example C.

DETAILED DESCRIPTION OF THE INVENTION

The resins disclosed are prepared by functionalizing copolymer beads, ina novel way, so as to improve diffusion of chemical species into theresin. During functionalization, weak-base exchange moieties aresubstituted onto the copolymer at haloalkylated sites which are mostaccessible to diffusion, while strong-base exchange moieties aresubstituted at haloalkylated sites which are least accessible todiffusion. The strong-base exchange moieties are more hydrophilic whencompared to the weak-base exchange moieties, which increases the waterretention capacity of the resin and improves diffusion of chemicalspecies into the resin. The improved diffusion promotes increasedexchange kinetics.

Partial functionalization with weak-base exchange moieties is preferablyachieved by contacting haloalkylated copolymer beads with a firstaminating agent. Contact is conducted under conditions such that theparticular functionalization reaction proceeds at a rate which is morerapid in comparison to the rate of diffusion for the first aminatingagent into the copolymer beads. In other words, the diffusion rate isthe limiting rate for the functionalization reaction. Thus, as the firstaminating agent diffuses into the bead, it reacts more readily withhaloalkylated sites located near the outer surfaces of the bead prior todiffusing further therein.

Due to the nearly quantitative nature of amination reactions, partialfunctionalization is preferably achieved by reacting the haloalkylatedcopolymer beads with a sub-stoichiometric amount of the first aminatingagent, which leaves unreacted those haloalkylated sites which are leastaccessible to diffusion. The resulting partially aminated resin beadsmay then be aminated with a tertiary amine so as to substitute at leasta portion of the remaining haloalkylated sites with strong-base exchangemoieties. Preferably, substantially all remaining haloalkylated sitesare substituted with the strong-base exchange moieties.

An equivalent method would be to partially haloalkylate the copolymerbeads in a manner such that the rate of haloalkylation is more rapid incomparison to the rate of diffusion for haloalkylating agents into thebeads. In such a method, haloalkylated sites are substituted at sites onthe copolymer which are most accessible to diffusion by thehaloalkylation reagents. Upon amination with the first aminating agent,weak-base exchange moieties are formed at such sites. Thereafter, anyremaining haloalkyl moieties may be reacted with a tertiary amine toform the strong-base exchange moieties, or the partially aminated beadsmay be further haloalkylated and the resulting haloalkyl sites convertedto strong-base exchange moieties.

Accordingly, suitable functionalization methods are those wherein therate of diffusion for weak-base functionalizing reagents into the beadsis limiting with respect to the particular functionalization reactioninvolved. Other methods will become evident to those skilled in the artin view of the disclosure herein.

The resins of the present invention are prepared by generally followingconventional methods, except that it is important to functionalize thecopolymer beads as previously described. Numerous weak- and strong-baseanion-exchange resins and methods for making them are generallydescribed in U.S. Pat. Nos. 2,642,417; 2,960,480: 2,597,492: 2,597,493:3,311,602; 2,632,000: 2,632,001 and 2,992,544, as well as by F.Helfferich in Ion Exchange, (McGraw-Hill 1962) at pps. 47-58, therelevant teachings of which are incorporated herein by reference.

In general, the resins are prepared by functionalizing a copolymer beadmatrix. The copolymer beads are normally prepared by suspensionpolymerization of a mixture which includes a monovinyl aromatic monomerlike styrene, a cross-linking monomer such as divinylbenzene, and aneffective amount of a free-radical polymerization initiator. Thereafter,the copolymer bead matrix is typically haloalkylated by reaction with ahaloalkylation agent, typically chloromethylmethylether, in the presenceof a Friedel-Crafts catalyst. After haloalkylation, the beads arepartially aminated with the first aminating agent, as definedhereinafter, to form the weak-base exchange moieties. Thereafter, thebeads are further aminated with a tertiary amine to form the strong-baseexchange moieties.

Suspension polymerization methods are well-known in the art. Suitablemethods include, for example, a single-stage polymerization process asdescribed by F. Helfferich in Ion Exchange, supra, at pages 35-36,wherein a single monomer mixture is suspension polymerized to producecopolymer beads. Also suitable is a "seeded" or multi-stagedpolymerization process described, for example, in U.S. Pat. Nos.4,419,245 and 4,564,644, the teachings of which are incorporated hereinby reference.

The monomers employed are addition polymerizable monovinyl aromaticcompounds and any addition polymerizable polyvinyl compound which mayact as a cross-linking monomer. Such monomers are well-known in the artand reference is made to Polymer Processes, edited by Calvin E.Schildknecht, published in 1956 by Interscience Publishers, Inc., NewYork, Chapter III, the relevant teachings of which are incorporated byreference. Of particular interest are water-insoluble monomers includingmonovinyl aromatics such as styrene, vinyl naphthalene,alkyl-substituted styrenes (particularly monoalkyl-substituted styrenessuch as vinyltoluene and ethylvinylbenzene) and halo-substitutedstyrenes such as bromostyrene, chlorostyrene, or vinylbenzylchloride,the polyvinyl aromatics such as divinylbenzene, divinyltoluene,divinylxylene, divinylnaphthalene, trivinylbenzene, divinyldiphenylether, and divinyldiphenylsulfone: and mixtures of one or more of themonomers. Preferred monomers include monovinyl aromatics like styrene,or a mixture of styrene with a monoalkyl- or halo-substituted styrenederivative, and polyvinyl aromatics like divinylbenzene. The mostpreferred monomers are styrene and divinylbenzene.

Copolymer beads are advantageously prepared from monomer mixtures havingfrom about 50 to about 99.9 weight percent, based on the weight ofmonomers employed, of the monovinyl aromatic monomer with the balance ofthe mixture, i.e., from about 0.1 to about 50 weight percent, being across-linking monomer. Preferred copolymer beads have from about 80 toabout 99 weight percent of the monovinyl aromatic monomer with fromabout 1 to about 20 weight percent being the cross-linking monomer.

A liquid diluent which is substantially inert under polymerizationconditions may be incorporated into the monomer mixture to obtainmacroporous copolymer beads. Suitable diluents, as known to those in theart, are organic liquids which are a solvent for the monomers employed,but a non-solvent for the resulting copolymer.

The term "macroporous" (also referred to as macroreticular) is widelyused in the art and, in general, refers to copolymer beads which haveregions of densely packed polymer chains exhibiting molecular-sizedporosity which are separated by copolymer-free voids, often referred toas mesopores (50-200 Å) and macropores (>200 Å). In contrast,microporous, or gel-type, resins have pores generally of molecular-size(generally less than about 50Å). Macroporous and gel resins are furtherdescribed in U.S. Pat. Nos. 4,224,415 and 4,382,124, the teachings ofwhich are incorporated herein by reference. Copolymer beads employedherein may be either macroporous or gel.

Methods for haloalkylating copolymer beads are well-known. Illustrativeof such are U.S. Pat. Nos. 2,642,417; 2,960,480: 2,597,492: 2,597,493;3,311,602; 2,632,000; 2,632,001 and 2,992,544 and Helfferich, supra, atpages 52-53, all of which have been incorporated herein by reference. Ingeneral, the copolymer beads are preferably haloalkylated by swellingthem with a haloalkylating agent and an effective amount of aFriedel-Crafts catalyst. Thereafter, the swollen copolymer beads areheated to a temperature sufficient to initiate reaction and thetemperature is maintained until obtaining a desired degree of reaction.The copolymer beads are preferably halomethylated and most preferablychloromethylated. Haloalkylated copolymer beads may also be obtained bycopolymerizing monomers like vinylbenzylchloride with a cross-linkingmonomer.

Weak-base exchange moieties are attached to the haloalkylated copolymerbeads by generally following methods discussed, for example, in thepatents identified in the preceding paragraph and by F. Helfferich inhis book, supra, at pages 53-58. In general, weak-base exchange moietiesmay be attached by heating haloalkylated copolymer beads in the presenceof a first aminating agent, as such compounds are described hereinafter.

Suitable first aminating agents are primary and secondary amines capableof reacting with the haloalkylated sites on the copolymer to formweak-base exchange moieties which are less hydrophilic relative to thestrong-base exchange moieties. First aminating agents desirablycorrespond to the formula:

    HNR.sup.1 R.sup.2

wherein:

R¹ is selected from hydrogen and aliphatic or aromatic groups having upto about 16 carbon atoms: and

R² is selected from aliphatic or aromatic groups having up to about 16carbon atoms.

Aliphatic groups preferably have from 1 to about 8 carbon atoms, andmore preferably from about 2 to about 6 carbon atoms. Examples ofpreferred aliphatic groups are alkyls like methyl, ethyl, propyl, orbutyl and hydroxyalkyls like methoxy, ethoxy, propoxy, and butoxy.

An aromatic group comprises either a single aromatic ring or a fusedaromatic ring system, such as in the case of a naphthalene ring. Anaromatic group optionally has an intermediate bridging moiety, forexample a --CH₂ -- moiety, which connects the nitrogen atom of the firstaminating agent with the aromatic ring or ring system. The aromatic ringor ring system may be carbocyclic or heterocyclic in nature and, ifheterocyclic, the hetero atom is preferably nitrogen. Aromatic groupspreferably have no more than about 8 carbon atoms. Examples of preferredaromatic rings are benzene or pyridine rings.

Examples of suitable first aminating agents are dimethylamine,diethylamine, dipropylamine, dibutylamine, methylamine, ethylamine,propylamine, butylamine, benzylamine, methylbenzylamine,N,N-dibenzylamine, aniline, methylaniline, or the aminopyridinecompounds discussed hereinafter.

Specifically contemplated herein are novel chelate-exchange resinsderived, in part, from aminopyridine compounds. Preparation of fullyfunctionalized resins with such aminopyridine compounds is disclosed inU.S. Pat. Nos. 4,031,038 and 4,098,867, the teachings of which areincorporated herein by reference. According to the present invention,the chelate-exchange resins have weak-base exchange moieties whichcorrespond to the formula: ##STR1## wherein: M is the copolymer beadmatrix:

Y is hydrogen or a C₁₋₄ alkyl;

Q is:

(1) --CH₂ --;

(2) --(C₂ H₃ R'NR')_(x) --CH₂ -- where each R' is hydrogen or methyl,and x is 1 or 2; or

(3) --NH-- or --C₂ H₄ --; and

R is:

(1) hydrogen, a C₁₋₄ alkyl, or C₂₋₄ hydroxyalkyl; and

when Q is --CH₂ --, R can also be:

(2) allyl, benzyl, or o-hydroxybenzyl;

(3) ##STR2## wherein each R' is hydrogen or methyl, and y is 0 or 1; (4)--(CH₂)_(m) OY where m is 2 or 3;

(5) --C₂ H₃ R'NR³ R⁴ where R' is hydrogen or methyl: R³ is hydrogen, aC₁₋₄ alkyl, a C₂₋₄ hydroxyalkyl, phenyl, or benzyl; and R⁴ is hydrogen,a C₁₋₄ alkyl, or a C₂₋₄ hydroxyalkyl;

(6) --C₂ H₄ SR" where R" is a C₁₋₄ alkyl;

(7) --C_(n) H_(2n) COOY where n is 1 or 2;

(8) --C_(n) H_(2n) SO₃ -- where n is 1 or 2; or

(9) CH₂ Z where Z is --CONH₂ or --NHCONH₂.

Preferred resins have chelate-exchange moieties which are derived from2-picolylamines. In such resins, the chelate-exchange moietiespreferably correspond to the formula: ##STR3## wherein M, R, R', y, andY are as previously defined. Examples of preferred 2-picolylamines are2-picolylamine, N-methyl-2-picolylamine, N-(2-hydroxyethyl)-2-picolylamine, N-(2-hydroxypropyl)-2-picolylamine, andbis-(2-picolyl)amine.

The copolymer beads are functionalized such that from about 15 to about95 percent of haloalkylated sites are functionalized with the weak-baseexchange moieties. A degree of functionalization below about 15 percentof available haloalkylated sites is undesired, because any increase inexchange kinetics is offset by limited weak-base exchange capacity.Above about 95 percent of available haloalkylated sites, the improvementin exchange kinetics is relatively small. Preferably from about 25 toabout 90 percent, more preferably from about 45 to about 85 percent, andmost preferably from about 50 to about 75 percent of availablehaloalkylated sites are functionalized with such moieties.

A swelling agent for the haloalkylated copolymer beads is not employedduring amination with the first aminating agent, as such swelling agentstend to promote relatively uniform amination of the beads. It isimportant, as previously discussed, to functionalize the beads in amanner such that only those haloalkylated sites most accessible todiffusion are functionalized with the weak-base exchange moieties.Accordingly, it is preferred to conduct amination with the firstaminating agent in a liquid medium which is substantially incapable ofswelling the haloalkylated copolymer beads, but miscible with the firstaminating reagent. Liquids which may be employed for this purpose arepolar liquids which do not react with the haloalkylated copolymer beadsor the first aminating agent. Examples of suitable polar liquids arewater, C₁₋₄ alcohols, or mixtures thereof. Water is a preferred liquidmedium.

Where the liquid medium is water, it preferably includes at least onesoluble salt which promotes functionalization of haloalkylated siteswhich are most accessible to diffusion. Many first aminating agents arehighly soluble in water and do not readily diffuse into thehaloalkylated copolymer beads. Addition of a soluble salt to the liquidmedium reduces the solubility of the first aminating agents in water andpromotes their diffusion into the beads. Examples of suitable solublesalts are alkali metal halides and alkali metal sulfates. A preferredsalt is sodium chloride due to relatively low cost. The amount employedwill vary depending upon the choice of soluble salt. Generally, anadequate amount of soluble salt is from about 100 grams per liter (g/l)up to saturation for the particular salt employed. For sodium chloride,an amount of from about 150 to about 280 g/l (a saturated solution) ispreferred.

Contact between the haloalkylated copolymer beads and the firstaminating agent occurs under conditions sufficient to react the firstaminating agent with the beads. Amination is preferably conducted at atemperature of from about 60° C. to about 100° C., and more preferablyfrom about 70° C. to about 90° C. for at least about 2 hours. A base,such as sodium hydroxide or sodium carbonate, is typically added withthe first aminating agent to minimize side reactions, a technique whichis known in the art.

After amination with the first aminating agent, the resulting partiallyaminated resin beads are further aminated with a tertiary amine toprovide hydrophilic, strong-base exchange moieties. Suitable tertiaryamines include, for example, trimethylamine, triethylamine,tripropylamine, tributylamine, dimethylethylamine,dimethylisopropanolamine, ethylmethylpropylamine, dimethylaminoethanol,diethylmethylamine, and dimethylethanolamine. A preferred tertiary amineis trimethylamine, due to relatively low cost, good availability, and asmall molecular size that allows for good penetration into the partiallyaminated resin beads.

Amination with the tertiary amine is achieved by contact with thepartially aminated resin beads under conditions sufficient to react theremaining haloalkylated sites, i.e., those sites which are leastaccessible to diffusion into the copolymer beads. The tertiary amine issuitably present in an amount sufficient to functionalize at least aportion of, and preferably substantially all, remaining availablehaloalkylated sites. The conditions include a temperature of from about20 to about 60° C. for least about 2 hours. Other conditions arewell-known in the art.

After amination, the resulting resin may be converted to its free-baseform by contact with a dilute, i.e., from about 0.5N to about 3N,aqueous base such as an alkali metal hydroxide solution. A preferredbase is a dilute aqueous sodium hydroxide solution.

In the case of gel copolymer beads, the weak-base exchange moieties aresubstituted at haloalkylated sites located substantially within acontinuous shell surrounding a central core that is substituted with thestrong-base exchange moieties. Such "core-shell" functionalizationoccurs due to the slow diffusion of the first aminating agent into thebead relative to the rate of reaction for such agents with thehaloalkylated copolymer beads. Thus, the core is least accessible todiffusion by the first aminating agents used to form the weak-baseexchange moieties.

Core-shell functionalization, in a strict sense, is not believed to beachieved in the case of haloalkylated, macroporous copolymer beads,since the greater porosity of such beads allows for some diffusion ofthe first aminating agent into interior portions thereof. However, acore-shell distribution of exchange moieties which is less defined isbelieved to occur in these beads. At any rate, similar kineticadvantages are realized by functionalizing macroporous beads such thathaloalkylated sites most accessible to diffusion are substituted withthe weak-base exchange moieties, while haloalkylated sites leastaccessible to diffusion are substituted with the strong-base exchangemoieties.

The resin suitably exhibits a water retention capacity of from about 30percent to about 60 percent while in the free-base form. Water retentioncapacity is determined by swelling a weighed amount of resin with water,removing excess water and then weighing the fully swollen resin. Theresin is then dried on a moisture balance until a constant weight isobtained. Water retention capacity is the ratio of water imbibed to thetotal combined weight of the resin plus imbibed water. Water retentioncapacity, on a substantially fully functionalized basis, isadvantageously at least about 30 percent. Preferred resins have a waterretention capacity of at least about 35 percent, more preferably atleast about 40 percent and most preferably from about 45 to about 55percent. As used hereinafter, water retention capacity is measured whilethe resin is in the free-base form, unless specified otherwise.

The resins preferably have a wet volume capacity of at least about 0.7meq/ml, more preferably at least about 0.8 meq/ml, and most preferablyat least about 0.9 meq/ml. Of this capacity, weak-base exchange moietiescontribute suitably from about 15 to 95 percent thereof, as previouslydescribed. Wet volume capacity may be determined by analyticaltechniques known in the art.

Resin particle size is not critical to obtain the benefits previouslydescribed herein. For most commercial applications, the resin preferablyhas a volume average particle diameter from about 100 to about 1,500,more preferably from about 150 to about 700, and most preferably fromabout 200 to about 600 micrometers (μm). Volume average particlediameter may be determined by any one of a number of commerciallyavailable instruments designed to make such measurements, such as aCriterion Model PC-320 Particle Size Analyzer available from theHIAC-Royco Company.

In the process of using the novel resins disclosed herein, chemicalspecies are separated from liquids by contact with the resin. Theprocess is characterized by exchange kinetics which are improved incomparison to resins functionalized only with the weak-base exchangemoieties. The term "exchange kinetics" as used herein refers to the rateat which chemical species are retained by the resin.

Suitable methods for conducting the separation are those resulting inintimate contact between the liquid and the resin. Examples of suitablemethods include fluidized beds, stirred tanks, batch tanks, andcocurrent or countercurrent flow columns. The contact may occurbatchwise, semi-batchwise, continuously, or semi-continuously.Preferably, the liquid is contacted with the resin in a continuoussystem employing a packed ion-exchange column.

The time required for contact will depend upon numerous factors, such asthe following: (1) the properties of the resin employed: (2) the amountof chemical species present in the liquid mixture: (3) the degree ofseparation desired: and (4) the amount of resin employed in the process.Thus, the time employed in most instances is more or less determinedempirically. Generally, a bed residence time of from about 0.1 hours (10bed volumes/hr) to about 10 hours (0.1 bed volume/hr), more preferablyabout 0.12 hours (8 bed volumes/hr) to about 1 hour (1 bed volume/hr),and most preferably about 0.17 hours (6 bed volumes/hr) to about 0.5hours (2 bed volumes/hr), yields acceptable results in a columnoperation. The term "bed volume" refers to a volume of the liquidmixture being treated which is equal to the volume of the resin employedin a resin bed.

The temperature at which the contact is conducted is one which does notadversely affect either the resin or the liquid being treated. Ingeneral, the temperature is limited only by the freezing point, boilingpoint, and viscosity of the liquid, as well as the temperature at whichthe components of the liquid or the resin itself begins to decompose. Ingeneral, temperatures from about 20° C. to about 100° C. are suitable.

The chelate-exchange resins of the present invention, as previouslydescribed, are particularly useful for recovery of heavy metal ionsdissolved in liquids, such as mine leachate solutions. Heavy metal ionsof interest are copper, nickel, iron, cobalt, silver, gold, mercury,platinum, vanadium, molybdenum, chromium, or cadmium, with copper andnickel being preferred metals. The liquid suitably has a pH of fromabout 1 to about 5, with the concentration of heavy metal ions beingfrom about 0.1 to about 10 g/l.

If desired, chemical species retained by the resin may be recovered byelution with a suitable regenerant. The regenerant employed and amountrequired will depend upon the particular resin and chemical speciesinvolved, as those skilled in the art can appreciate. Where heavy metalsare retained by the chelate resins previously described, suitableregenerants are aqueous solutions of strong inorganic acids and ammoniumhydroxide. Preferred regenerants are sulfuric acid and ammoniumhydroxide.

The concentration of the inorganic acid is important to obtain areasonably quick elution of retained heavy metal ions. Generally, aconcentration of from about 0.1N to about 5N provides a reasonably sharpand quick elution. Below about 0.1N, the retained heavy metal ions arenot as easily eluted from the resin and the elution is not as sharplydefined. Concentration is not as important for the ammonium hydroxidesolution. An ammonium hydroxide concentration of from about 0.001N toabout 15N is adequate.

The eluate obtained will generally have a greater concentration of thechemical species, such as the heavy metal ions, in comparison with theliquid being treated. Where heavy metal ions are in the eluate, they maybe further recovered by conventional methods, such as electrowinning,crystallization, precipitation, or cementation.

SPECIFIC EMBODIMENTS OF THE INVENTION

The following examples illustrate the present invention and should notbe construed, by implication or otherwise, as limiting the scope of theappended claims. All parts and percentages are by weight and alltemperatures in degrees Celsius (°C.) unless indicated otherwise.

EXAMPLE 1

This example concerns preparation of a chelate-exchange resin andevaluation of its exchange kinetics with respect to removing copper ionsfrom an acidified, aqueous solution.

Macroporous copolymer beads are prepared by polymerizing styrene, acommercially available divinylbenzene mixture, and2,2,4-trimethylpentane as a liquid diluent, in a single-stage suspensionpolymerization described in U.S. Pat. No. 3,637,535. The diluent isemployed in an amount sufficient to yield an organic phase having 42weight percent diluent, based upon the weight of the monomers anddiluent. The commercially available divinylbenzene mixture is obtainedfrom The Dow Chemical Company and consists of 55 weight percentdivinylbenzene, with the balance of the mixture being essentiallyethylvinylbenzene. The resulting macroporous copolymer beads have 6weight percent divinylbenzene, based on total weight of the monomersemployed and have a volume average diameter of 460 μm.

The copolymer beads are chloromethylated by first adding 100 grams ofthe beads and 500 grams of chloromethylmethylether to a 1 liter,three-necked, round-bottomed flask. The flask is equipped with anoverhead mechanical stirrer, an addition funnel, and a condenserconnected to a caustic scrubber. The flask contents are then agitatedfor 30 minutes while the copolymer beads are allowed to swell.Thereafter, a 30 gram portion of ferric chloride, a Friedel-Craftscatalyst, is added to the flask. The flask contents are heated to atemperature of 50° C. which is maintained for about 3 hours. Afterallowing the flask contents to cool, the reaction is quenched with 500milliliters (ml) of methanol. The resulting chloromethylated copolymerbeads are recovered from the flask and washed a final time withmethanol.

The chloromethylated copolymer beads are partially aminated with anaminopyridine compound. A 500 ml, three-necked, round-bottom flaskequipped with an overhead mechanical stirrer, a thermowell, and awater-cooled condenser is charged with 63 grams (0.30 equivalents) ofthe chloromethylated beads, 29 grams (0.15 equivalents) ofN-(2-hydroxypropyl)-2-picolylamine which is obtained from the Rilley Tar& Chemical Company, 12 grams (0.15 equivalents) of an aqueous 50 percentsodium hydroxide solution, 114 grams of sodium chloride and 300milliliters of water. The flask contents are thereafter heated to 85° C.and maintained at this temperature with agitation for three hours. Atthis point, the liquid in the flask exhibits a faint yellow tint and hasa neutral pH. The flask contents are allowed to cool and the partiallyaminated resin beads are recovered from the liquid. After the abovepartial amination, 48 percent of available chloromethyl sites arereacted.

Prior to contacting the partially aminated resin beads with a tertiaryamine, the beads are treated with 300 ml of an aqueous 5 percentsolution of hydrochloric acid for 10 minutes. Treatment with dilute acidexpands the copolymer structure and promotes removal of residual amountsof reactants from the bead. The beads are removed from the dilute acidsolution and treated with an excess amount of an aqueous 4 percentsolution of sodium hydroxide for 30 minutes. The beads are then washedto yield 174 ml of partially aminated resin beads exhibiting a waterretention capacity of 36 percent and a wet volume eapacity of 0.74meq/ml.

Amination is substantially completed by reacting remaining chloromethylgroups with trimethylamine. A 68 ml portion of the partially aminatedresin beads is placed with 120 ml of an aqueous 24 percenttrimethylamine solution in a 500 ml Erlenmeyer flask. The flask contentsare swirled and allowed to stand at ambient temperature, i.e.,approximately 23° C., for 10 hours. The beads are filtered from theliquid and washed with 200 ml of an aqueous five percent solution ofhydrochloric acid for 20 minutes and then 500 ml of a 1N aqueous sodiumhydroxide solution for 20 minutes. Thereafter, the beads are washed withwater to remove residual traces of salts. The resulting chelate-exchangeresin beads exhibit a water retention capacity of 46 percent and a wetvolume capacity of 0.98 meq/ml. The beads have a core of strong-base,quaternary ammonium functional groups represented by: ##STR4## and anouter shell of weak base functional groups represented by: ##STR5##wherein M, in both formulas, represents the copolymer bead matrix.

The resin exchange kinetics are demonstrated by removal of copper ionsfrom an acidified, aqueous solution having about 6 g/l of copperdissolved therein. The copper solution is prepared by dissolving 23.5grams of copper sulfate pentahydrate in 100 ml of deionized water andthereafter adding a sufficient amount of deionized water to make up avolume of 900 ml. The solution is then adjusted to a pH of 2 by adding asufficient amount of a 1.0N aqueous hydrochloric acid solution and thenthe solution is finally diluted with water to 1 liter in volume. An 8 mlportion of the chelate-exchange resin beads, as previously described, isplaced in a flask with 200 ml of the copper solution. The flask contentsare magnetically stirred at ambient temperature, i.e., about 23° C.

A two ml sample of the copper solution is taken periodically from theflask, generally once about every 5-10 minutes, and analyzed by visuallight spectrophotometry to determine the amount of copper being removedby the resin beads. A one ml aliquot of each sample is diluted withthree ml of an aqueous 28 percent ammonium hydroxide solution. Afterdilution, the absorbance at 610 NM is measured using a one centimeterflow-through cell in a Perkin-Elmer Lambda 4B UV/VIS spectrometer.Conversion ("X") is calculated by: ##EQU1## where: A_(o) is absorbanceat time t=0;

A_(t) is absorbance at time t=t; and

A_(eq) is absorbance at time t=24 hours.

Copper retention reaches an equilibrium level well in advance of 24hours and, therefore, the solution is sampled and analyzed at a time of24 hours to obtain an absorbance reading representing the equilibriumcapacity for the resin.

The results are illustrated by FIG. 1 which is a graph of conversionversus time. The circles indicate data points for Example 1. The timenecessary to reach one-half of the equilibrium capacity ("T_(1/2) ") isdetermined to be 5.6 minutes.

COMPARATIVE EXAMPLE A

This example concerns preparation of a chelate-exchange resin which issubstantially completely functionalized withN-(2-hydroxypropyl)-2-picolylamine. The procedure followed issubstantially similar to that employed for Example 1, except asspecified hereinafter.

A 250 ml, three-necked, round-bottom flask is charged with 32.3 grams(0.15 equivalents) of the chloromethylated copolymer beads of Example 1,32.5 grams (0.18 equivalents) of N-(2-hydroxypropyl)-2-picolylamine, 13grams (0.16 equivalents) of an aqueous 50 percent sodium hydroxidesolution, 57 grams of sodium chloride, and 150 ml of water. The flask isequipped with an overhead mechanical stirrer, a thermowell, and awater-cooled condenser. The flask contents are stirred and heated to atemperature of 85° C. for 3 hours. All remaining procedures aresubstantially similar to Example 1. The resin has a water retentioncapacity of 34.5 percent in the free-base form and a wet volume capacityof 1.045 meq/ml.

The exchange kinetics for the resin are determined as in Example 1 andthe data is shown in FIG. 1 for comparison therewith. The squaresidentify data points for Comparative Example A. The resin beads exhibita T_(1/2) of 16.1 minutes. Comparison of this T_(1/2) value with theresult from Example 1 indicates that exchange equilibrium occurs moreslowly when the resin is substantially completely functionalized withweak-base exchange moieties, as evidenced by the significantly largerT_(1/2) value. Thus, the data of Example 1 indicates improved exchangekinetics due to a quicker approach toward exchange equilibrium withrespect to copper removal.

EXAMPLE 2

The procedure of Example 1 is repeated using bis-(2-picolyl)amine tofunctionalize the chloromethylated copolymer beads. The proceduresemployed are substantially similar to Example 1, except as indicatedotherwise hereinafter.

A 1 liter round-bottom flask is initially charged with 252 grams (1.2equivalents) of the chloromethylated beads, 120 grams (0.6 equivalents)of bis-(2-picolyl)amine purchased from the Rilley Tar & ChemicalCompany, 120 grams (0.6 equivalents) of the aqueous 50 percent sodiumhydroxide solution, and 500 ml of a saturated 26 weight percent sodiumchloride solution. The flask contents are heated to 85° C. andmaintained at this temperature with agitation for three hours topartially aminate the beads. After partial amination, 49 percent ofavailable chloromethyl sites are reacted.

A 70 ml portion of the partially aminated beads is then placed with 120ml of the aqueous 24 percent trimethylamine solution in a 500 mlErlenmeyer flask. The flask contents are swirled and allowed to stand atambient temperature, i.e., approximately 23° C., for 10 hours. Theresulting chelate-exchange resin beads exhibit a water retentioncapacity of 46 percent and have a wet volume capacity of 1.05 meq/ml.The beads have a core of strong-base, quaternary ammonium functionalgroups represented by: ##STR6## and an outer shell of weak-basefunctional groups represented by: ##STR7## wherein M, in both formulas,represents the copolymer bead matrix.

The exchange kinetics for the resin are determined as in Example 1. Theresults are illustrated graphically by FIG. 2. The circles identify datapoints for Example 2. The resin beads exhibit a T_(1/2) of 6.6 minutes.

COMPARATIVE EXAMPLE B

Example 2 is repeated, except that the chloromethylated copolymer beadsare substantially completely functionalized with bis-(2-picolyl)amine.The procedure followed is substantially similar to Example 2, except asindicated otherwise hereinafter.

The flask is initially charged with 39 grams (0.187 equivalents) of thechloromethylated copolymer beads, 40 grams (0.2 equivalents) ofbis-(2-picolyl)amine, 18.8 grams (0.235 equivalents) of the aqueous 50percent sodium hydroxide solution, and 170 ml of the saturated sodiumchloride solution. All other procedures are substantially the same as inExample 2. The resin has a water retention capacity of 39 percent andwet volume capacity of 1.11 meq/ml. The exchange kinetics are determinedas in Example 2 and the results are graphed on FIG. 2 for comparisontherewith. The squares indicate data points for Comparative Example B.The resin beads exhibit a T_(1/2) of 10.1 minutes, thereby indicatingdecreased exchange kinetics when compared to the smaller T_(1/2)associated with the resins of Example 2.

EXAMPLE 3

The procedure of Example 1 is substantially repeated, except that thechloromethylated copolymer beads are partially aminated withN-(2-hydroxypropyl)-2-picolylamine to a greater extent prior toamination with trimethylamine. The procedure followed is substantiallysimilar, except as indicated otherwise hereinafter.

The flask is initially charged with 47.4 grams (0.22 equivalents) of thechloromethylated beads, 41 grams (0.247 equivalents) ofN-(2-hydroxypropyl)-2-picolylamine, 25 grams (0.315 equivalents) of thesodium hydroxide solution, and 500 ml of a saturated 26 weight percentsodium chloride solution. After partial amination as in Example 1, 87percent of available chloromethyl sites are reacted.

Thereafter, a 70 ml portion of the partially aminated beads is placedwith 120 ml of the aqueous 24 percent trimethylamine solution in a 500ml Erlenmeyer flask. The flask contents are swirled and allowed to standat ambient temperature, i.e., approximately 23° C., for 10 hours. Allother procedures are substantially similar to those of Example 1. Theresin has a water retention capacity of 36 percent and a wet volumecapacity of 0.98 meq/ml. The exchange kinetics are determined as inExample 1 and the results are graphed in FIG. 3. The circles indicatedata points for Example 3. The resin beads exhibit a T_(1/2) of 53.2minutes.

COMPARATIVE EXAMPLE C

The procedure of Example 3 is substantially repeated, except that thepartially aminated resin beads are not aminated with the trimethylaminesolution. The resin has a water retention capacity of 28 percent and awet volume capacity of 0.99 meq/ml. The exchange kinetics are determinedas in Example 1. The results are graphed in FIG. 3 for comparisontherewith. The resin beads exhibit a T_(1/2) of 83.1 minutes, therebyindicating that exchange kinetics are substantially improved, aspreviously described in the foregoing examples, through formation ofhydrophilic, strong-base exchange moieties at chloromethylated siteswhich are least accessible to diffusion.

What is claimed is:
 1. A process for preparing resin beads whichcomprise cross-linked copolymer beads having weak-base exchange moietiessubstituted at haloalkylated sites which are most accessible todiffusion and hydrophilic, strong-base exchange moieties substituted athaloalkylated sites which are least accessible to diffusion, the processcomprising the steps of:(a) contacting the haloalkylated, cross-linkedcopolymer beads with a first aminating agent under conditions and in anamount sufficient to substitute at least a portion of the haloalkylatedsites with the weak-base exchange moieties and obtain partiallyfunctionalized copolymer beads, the substitution being conducted at areaction rate which is greater than the rate at which the firstaminating agent diffuses into the copolymer beads: and (b) contactingthe partially functionalized copolymer beads with a tertiary amine underconditions and in an amount sufficient to functionalize at least aportion of remaining haloalkylated sites with the strong-base exchangemoieties.
 2. The process of claim 1 wherein the first aminating agentcorresponds to the formula:

    HNR.sup.1 R.sup.2

wherein: R¹ is hydrogen, an aliphatic group having up to about 16 carbonatoms, or an aromatic group having up to about 16 carbon atoms: and R²is an aliphatic group having up to about 16 carbon atoms or an aromaticgroup having up to about 16 carbon atoms.
 3. The process of claim 1wherein the first aminating agent is dimethylamine, diethylamine,dipropylamine, dibutylamine, methylamine, ethylamine, propylamine,butylamine, benzylamine, methylbenzylamine, N,N-dibenzylamine, aniline,methylaniline, or an aminopyridine.
 4. The resin of claim 1 wherein thefirst aminating agent is 2-picolylamine, N-methyl-2-picolylamine,N-(2-hydroxyethyl)-2-picolylamine, N-(2-hydroxypropyl)-2-picolylamine,or bis-(2-picolyl)amine.
 5. The process of claim 14 wherein thecross-linked copolymer beads are chloromethylated.
 6. The process ofclaim 1 wherein step (b) further comprises functionalizing substantiallyall remaining haloalkylated sites with the strong-base exchangemoieties.
 7. The process of claim 1 wherein the copolymer beads aremacroporous.
 8. The process of claim 1 wherein the copolymer beads areof a gel copolymer.
 9. The process of claim 1 wherein the copolymerbeads comprise from about 50 to about 99.9 weight percent of a monovinylaromatic monomer and from about 0.1 to about 50 weight percent of across-linking monomer based on the weight of the monomers.
 10. Theprocess of claim 9 wherein the monovinyl aromatic monomer is styrene.11. The process of claim 9 wherein the cross-linking monomer isdivinylbenzene.
 12. The process of claim 1 wherein from about 15 toabout 95 percent of the haloalkylated sites are substituted with theweak-base exchange moieties.
 13. The process of claim 1 wherein step (a)further comprises contacting the haloalkylated cross-linked copolymerbeads with the first aminating agent in a liquid medium, the liquidmedium being substantially incapable of swelling the cross-linkedcopolymer beads, but miscible with the first aminating agent.
 14. Theprocess of claim 13 wherein the liquid medium is water, a C₁₋₄ alcohol,or mixtures thereof.
 15. The process of claim 13 wherein the liquidmedium is water.
 16. The process of claim 15 wherein the liquid mediumcomprises at least one soluble salt selected from alkali metal halidesand alkali metal sulfates.
 17. The process of claim 16 wherein thesoluble salt is sodium chloride.
 18. The process of claim 16 wherein thesoluble salt is present in an amount of from about 100 g/l up tosaturation for the soluble salt in water.
 19. The process of claim 1wherein the tertiary amine is trimethylamine, triethylamine,tripropylamine, tributylamine, dimethylethylamine,dimethylisopropanolamine, ethylmethylpropylamine, dimethylaminoethanol,dimethylethanolamine, or diethylmethylamine.
 20. The process of claim 1wherein the tertiary amine is trimethylamine.